Click to viewOne of the many mysteries of Mars is how the planet lost its magnetic field 4 billion years ago. Evidence suggests the planet once had a magnetic field just like Earth's, created by a churning molten dynamo in the planetary core. But what could have caused that core to stop spinning, and stop generating a magnetic field, over a period of a mere few millennia? A group of geophysicists may have the answer: a massive meteor impact.

Scientists have determined that the Martian magnetic field might have been maintained by an asteroid in orbit around the planet — an asteroid that eventually orbited a bit too close to Mars and crashed into the planet. According to New Scientist:

Now Jafar Arkani-Hamed of the University of Toronto, Canada, and colleagues say the gravitational tug of an orbiting asteroid may have powered a dynamo by pulling on the fluid in Mars's core. The team's lab and model simulations showed that an asteroid orbiting 75,000 kilometres above Mars could have maintained a dynamo for 400 million years, before the rock crashed into the planet and switched it off.

Other researchers in the field are skeptical, saying that an asteroid in orbit wouldn't have had enough energy to start up that dynamo in the first place.